1. Field of the Invention
The present invention pertains to automatic direction control systems for controlling the radiating direction of seismic sources and more particularly to an automatic direction control system for omni-directional shear wave sources.
2. Related Prior Art
Prior art has discussed many ways to direct the seismic energy produced in currently available seismic sources. The following patents illustrate several of the known and commonly used methods for detecting location and thereby directing energy from a source. In general, the following patents relate to systems for detecting and correcting heading errors in marine or other types of navigation systems.
U.S. Pat. No. 4,564,909, "Error Sensing System for Vessels With Absolute Zero Referencing", issued to Melvin G. Kramer, relates to an error sensing system for use in marine autopilot systems of the type having a photographic film compass disk aligned with the direction North and sensing means associated with the disk is operative to sense relative movement between the disk and the sensors in response to changes in the heading of the vessel. A first direction error surface portion on the disk has opaque and transparent areas defining a zero reference therebetween, a second magnitude error surface portion on the disk has a second zero reference position in predetermined relation to the first zero reference position and an area of variable transparency extending along the second surface portion. The first and second surface portions are defined by images along the peripheral surface of the photographic film disk. The disk sensing system has first and second light sensors aligned with the first and second surface portions, respectively, to generate direction error and magnitude error signals representative of the direction and extent of movement of the vessel away from a given heading.
U.S. Pat. No. 3,772,503, "Course Difference Logic for Use with Digital Magnetic Compass", issued to John T. Fowler, relates to an electronic circuit having logic circuitry for use with a digital compass for providing an output indication of the difference between an actual and an intended course. Digital signals provided by a digital compass are processed in conjunction with a digital signal representation of intended course position to provide an output signal representing course difference and which can be employed by display or automatic pilot apparatus.
U.S. Pat. No. 3,746,842, "Digital Magnetic Compass", issued to John T. Fowler, relates to a magnetic compass in which the compass card and gimbal mounting are all located inside a sealed enclosure filled with damping fluid. The compass card is digitally coded and photoelectric means are provided for reading the card and generating a digital number corresponding to the rotational position of the card relative to the enclosure, corrected for local magnetic variation. Means are provided for comparing the compass reading with the corrected course reading to provide an error signal or an off-course alarm if desired.
U.S. Pat. No. 4,179,741, "Magnetic Compasses", issued to Aldo Rossani, relates to a static compass capable to supply a digital indication of the magnetic head angle, comprising circuits for supplying two signals corresponding to two orthogonal components of the horizontal component of the earth magnetic field, said signals consisting of a carrier frequency, amplitude modulated according to the value of said orthogonal components, a circuit for introducing a phase shift through 90.degree. 18 electric on one of said modulated frequencies, a circuit effecting the sum and the difference of said two signals, a circuit for measuring the difference of phase between said sum and difference signals and for introducing count pulses in a number proportional to said difference of phase in order to drive digital counters associated to digital display elements showing the head angle with respect to the horizontal component of the earth magnetic field, further including apparatus for indicating the error with respect to a selected head angle, said apparatus including an arrangement for manually entering and memorizing said head angle, a counter predisposed for counting in amplitude and sign the error between the measured head angle and the selected angle, and an analogic/digital display arranged so as to indicate the amount, the direction and the variation in time of the error between the measured orientation and the orientation as set and memorized.